Casters and ranged

While gearing a Caster you will need to reach your Hit cap, wich is 18% for magical spells.
(Since a spell cannot be parried, you will ofcourse not need any Expertise.)

Most Caster classes gets alot of Hit from talents and active buffs (like shadow priest talent Misery, and Moonkins' Faerie Fire gives 3% for example). Draenei racial also gives 1% Hit.

In PvP you would want to have Spell Penetration also, wich reduces targets resistances. This is not needed in PvE though, since mobs and bosses very rarely or never got any resistances.

For Hunters, wich is a Ranged but not a magical class, you need 8% Hit. Even though their attacks are Physical you do not need any Expertise, since ranged attacks cannot be Parried or Dodged.

More than reaching the Hit cap, you should be gearing towards your main stat wich in most cases will be Spell Power. Haste benefits most classes very well also. Look up theorycraft forums for what your class needs (someone can add links or a post of their own in this forum also.)

Without any gear, buffs or talents a level 80 caster requires 446 hit rating for spells to never miss against a level 83 boss (raid boss). With a 446 hit rating (17%) raid bosses will resist your spells 0% of the time unless they have special resistances against your school of magic, such as shadow.

As stated before, the highest level 83 mob currently present is level 83 (e.g. all WotLK raid bosses). +17% spell hit rating is the most that will be effective against them.

You have at your disposal a variety of means to achieve the 446 hit rating:
1. Gear.
2. Gems.
3. Talent tree.
4. Balance Druid - the balance druid has a buff, Improved Faerie Fire, which adds 3% to every raid member's hit. Does not stack with a shadow priest's Misery.
5. Shadow Priest - when this class activates the raid-wide buff Misery, everyone gains 3% to hit. Does not stack with balance druid's Improved Faerie Fire.
6. Draenei - characters of this race give a 1% to hit buff to all party members. This ability does stack with Improved Faerie Fire and Misery but is not raid-wide.
7. Food! - some buff food will give you an additional 40 hit rating for 1 hour.

Hope this helps!